Amorphous resins such as polymethyl methacrylate (hereinafter referred to as “PMMA”) and polycarbonate (hereinafter referred to as “PC”) are used in extensive fields such as optical materials and various parts of household electric appliances, OA apparatuses and automobiles owing to their transparency and dimensional stability.
In recent years, these resins are widely used also especially for optical materials with higher performance such as optical lenses, prisms, mirrors, optical discs, optical fibers, liquid crystal display sheets and films, and light guide plates. These resins are required to have higher optical properties, moldability and heat resistance.
Further presently, these transparent resins are also used as illumination members of automobiles and the like such as tail lamps and head lamps. In recent years, for expanding the car interior space and reducing the gasoline consumption, there are such tendencies as to shorten the distances between light sources and tail lamp lenses, inner lenses and other various lenses of head lamps, shield beams and the like and also to thin parts. These resins are required to have excellent moldability. Furthermore, since motor vehicles are used in severe conditions, the resins are required to little change in shape at high temperature and humidity and also to be excellent in flaw resistance, weather resistance and oil resistance.
However, the PMMA resin has a problem of insufficient heat resistance, though it is excellent in transparency and weather resistance. On the other hand, the PC resin has such problems that since it is large in birefringence, namely, optical strain, the PC resin molding is optically anisotropic and that the resin is remarkably inferior in moldability, flaw resistance and oil resistance, though the resin is excellent in heat resistance and impact resistance.
So, for the purpose of improving the heat resistance of PMMA, resins containing a maleimide-based monomer, maleic anhydride monomer or the like as an ingredient for imparting heat resistance are developed. However, these resins have such problems that the maleimide-based monomer is expensive and low in reactivity and that maleic anhydride is low in thermal stability.
As methods for solving these problems, copolymers containing glutaric anhydride units obtained by heating a copolymer containing unsaturated carboxylic acid monomer units using an extruder for performing cyclization reaction are disclosed, for example, in Patent Documents 1 and 2. However, copolymers containing glutaric anhydride units obtained by heat-treating said copolymer by a suspension polymerization method or emulsion polymerization method have a problem that high colorless transparency cannot be obtained due to the foreign matter generated by the polymerization method.
Therefore, if bulk polymerization or solution polymerization not using a so-called polymerization aid such as a dispersing agent or emulsifying agent can be applied as a polymerization method for producing a precursor copolymer containing unsaturated carboxylic acid units, it can be expected that a copolymer with the excellent high colorless transparency required for optical materials can be obtained, and further a continuous polymerization process, if employed allows the copolymer composition and molecular weight distribution to be controlled. Thus, intensive studies have been made in this regard.
For example, processes comprising the steps of producing a copolymer containing unsaturated carboxylic acid alkyl ester units and unsaturated carboxylic acid units by bulk polymerization or solution polymerization, in succession, heating the obtained polymerization solution, for separating and removing the unreactive monomers and the unreactive monomers and the solvent, and further heating said copolymer for performing cyclization reaction are disclosed in Patent Documents 3 and 4 as methods for obtaining said copolymer containing glutaric anhydride units by a bulk polymerization or solution polymerization method.
However, though a polymerization method is described in the production process disclosed in Patent Document 3 (see Examples), the document does not particularly describes the devolatilization or cyclization of the obtained polymerization solution (see “Detailed Explanation of the Invention” and Examples). Further, there are such problems that in the case where said polymerization solution is heat-treated in the “high temperature vacuum chamber” described in the examples, a long-time heat treatment at a high temperature is required to consume enormous labor and energy for perfectly removing the unreactive monomers and the solvent in vacuum and for completing the cyclization reaction, and further that the obtained copolymer containing glutaric anhydride units is remarkably colored.
Furthermore, Patent Document 4 discloses a production process in which the copolymer solution (a) obtained by polymerization reaction is continuously supplied into a devolatilization tank for performing devolatilization and cyclization reaction. Also in this case, there are such problems that a long-time heat treatment at a high temperature is required to consume enormous labor and energy for perfectly removing the unreactive monomers and the solvent in vacuum and for completing the cyclization reaction, and further that the obtained copolymer containing glutaric anhydride units is remarkably colored.
Moreover, Patent Document 5 discloses a solution polymerization process for producing a copolymer containing unsaturated carboxylic acid alkyl ester units and unsaturated carboxylic acid units, but does not discuss the devolatilization or cyclization reaction using the polymerization solution.
In these situations, the present inventors proposed a process for producing a glutaric anhydride-containing copolymer excellent in colorless transparency and retention stability by producing a copolymer containing specific unsaturated carboxylic acid units under specific polymerization conditions and in succession heat-treating said copolymer, as described in Patent Document 6
This proposed technique could significantly improve the coloration and retention stability of the glutaric anhydride-containing copolymer obtained by performing continuous polymerization at a specific polymerization temperature. However, even the method of performing devolatilization and cyclization reaction using a double screw extruder disclosed in Patent Document 6 has a problem of low productivity since it is necessary to perform heat treatment at a high temperature for a long time by such methods as using an extruder with a very high ratio (L/D) of screw length to screw diameter and controlling the supplied amount of the copolymer for securing a retention time, in order to perfectly remove the unreactive monomers and the solvent in vacuum and to complete the cyclization reaction. At the same time, there is a problem that conspicuous coloration occurs due to the thermal decomposition of polymer chains caused by the heat generated by shearing in the case where the reaction mixture is retained in the double screw extruder for a long time.
That is, the resins that could be used for optical materials having higher performance such as optical lenses, prisms, mirrors, optical discs, optical fibers, liquid display sheets and films and light guide plates could not be produced by the production processes disclosed in Patent Documents 3 through 6. A process capable of industrially advantageously producing a copolymer containing glutaric anhydride units with higher colorless transparency and excellent thermal stability is desired.
[Patent Document 1] JP49-85184A (Pages 1-2, Examples)
[Patent Document 2] JP01-103612A (Pages 1-2, Examples)
[Patent Document 3] JP58-217501A (Pages 1-2, Examples)
[Patent Document 4] JP60-120707A (Pages 1-2, Examples)
[Patent Document 5] JP06-049131A (Pages 1-2, Examples)
[Patent Document 6] JP2004-002711A (Pages 1-2, Examples)